Its says it now has the power to block tweets in a specific country if the government legally requires it to do so. Dissidents and activists fear the new policy will stifle free speech.

Reporting from San Francisco — Twitter has promoted itself as a beacon of free speech, and that image was burnished when revolutionaries used the social media service to organize protests during last year's Arab Spring uprising.

But in what many view as an about-face, Twitter now says it has the power to block tweets in a specific country if the government legally requires it to do so, triggering outrage around the world, especially in Arab countries.

Dissidents and activists there fear the new policy will stifle free speech and thousands of users are threatening to boycott Twitter.

"Is it safe to say that Twitter is selling us out?" asked Egyptian activist Mahmoud Salem

The flood of criticism was unusual for Twitter, which drapes itself in the 1st Amendment. Its chief executive, Dick Costolo, refers to it as "the free speech wing of the free speech party."

Jack Dorsey, who created Twitter, even named one of the conference rooms at his San Francisco company "Tahrir Square" in recognition of the pivotal role that Twitter played in the uprising in Cairo.

But Twitter, like other major Internet companies, is struggling to reconcile its philosophical opposition to censorship with the economic desire to fan out around the globe.

Facebook, Google and Yahoo navigate a complex web of laws and state-imposed restrictions that can be used to suppress dissident voices and sway public opinion. It is common practice for Internet companies to take down content that is illegal in a particular country.

Twitter insists that it remains fully committed to free speech. It used to be that if Twitter removed a tweet, it vanished from the Web. Now a tweet that violates the law in one country will still be visible in the rest of the world.

Twitter will post a censorship notice whenever a tweet is removed, similar to what Google does. It will share the removal requests on the Chilling Effects website, which advocates for Internet freedom and tracks take-down notices.

Twitter said it would not remove any tweets unless it is legally required to do so, and then only after an internal review.

Twitter's general counsel, Alexander Macgillivray, a former Google lawyer who helped the Internet search giant craft its censorship policies, also helped create the chillingeffects.org website while at Harvard.

Yet when legally required, Twitter has removed tweets that infringed on copyrights or link to child pornography.

It says it has endeavored to be transparent. Twitter publicly disclosed that the U.S. government had obtained a court order requiring Twitter to hand over information about four Twitter users in the WikiLeaks investigation. Twitter said it went public so that the users could fight the request. It says it's applying that same principle here.

"One of our core values as a company is to defend and respect each user's voice," Twitter wrote in a blog post. "We try to keep content up wherever and whenever we can, and we will be transparent with users when we can't."

Some free-speech advocates defended Twitter, saying it was handing them tools to fight censorship.

Zeynep Tufekci, assistant professor at the University of North Carolina and a fellow at the Harvard Berkman Center for Internet and Society, said she found herself in the unusual position of praising, not condemning, the policies of an Internet company.

"Twitter is setting the bar as high as it can," Tufekci said. "It does not deserve the reaction it's getting."

Said Jillian York, director for international freedom of expression at the Electronic Frontier Foundation: "Once people see how Twitter is implementing this, they will calm down."

But other groups accused Twitter of siding with censors and demanded that it scrap the new policy.

"Twitter is depriving cyber dissidents in repressive countries of a crucial tool for information and organization," Reporters Without Borders, a journalist organization, wrote in a letter to Dorsey, Twitter's executive chairman.

State Department spokeswoman Victoria Nuland on Friday credited Twitter with being transparent about its approach to censorship, but said it was too early to tell if policy would harm users.

Twitter's medium for lightning-quick self-expression has powered political protests throughout the world from the Occupy Wall Street movement in the United States to the Arab Spring uprisings in Egypt, Bahrain, Tunisia and Syria.

Nowhere was it a more important tactical tool than in the uprising that overthrew Egyptian President Hosni Mubarak.

"Twitter had a massive effect on the course of the revolution before, during and after," said Ahmad Saied, an Egyptian journalist and a blogger.

Its ambitions to grow from 100 million active users to more than 1 billion may ultimately bring Twitter into conflict with its ideals.

With its expansion into more countries will come increased pressure to censor tweets. If it violates the law in a country where it has employees, those employees risk arrest and prosecution. That includes democracies such as France and Germany, which have strict prohibitions on Nazi propaganda.

"They've said their intention is to remain supportive of free speech and to continue to enable people to use Twitter as a tool for organizing and communication," said Rebecca MacKinnon, a fellow at the New America foundation who follows freedom of expression online. "But we shouldn't take them at their word. We should look at what they do and hold them to their word."